1. Field of the Invention
The present invention relates to an apparatus for holding a lens barrel for a projection optical system used in an exposing apparatus or the like for the manufacture of semiconductive elements.
2. Description of the Related Art
In a projection exposing apparatus for manufacturing semiconductive elements, the image of a pattern of a mask is projected onto a photosensitive substrate (e.g., a wafer) maintained on a stage through a projection optical system. The stage for holding the wafer is held on a fixed board (i.e., a base portion).
The projection optical system includes a refracting system, a reflecting system or a catadioptric system comprising a combination of the two, and also includes a plurality of optical elements (e.g., a lens, a mirror, a concave mirror, etc.). Usually, a lens barrel for holding the projection optical system is fixed to a lens barrel holding bed, which is coupled to the aforementioned base portion so as to be spaced apart by a predetermined spacing in the direction of the optical axis of the projection optical system. The structure of such a projection exposing apparatus is disclosed, for example, in U.S. Pat. 5,137,349.
FIGS. 5A and 5B of the accompanying drawings show a prior art apparatus for fixing the lens barrel of a projection optical system to a lens barrel holding bed. In particular, the lens barrel 40 is provided with a projection optical system. Although only one lens L is shown in FIG. 5A, the lens barrel is actually provided with a plurality of lenses. Furthermore, lens barrel 40 is formed with a flange portion 44 substantially orthogonal to the optical axis 41 of the projection optical system and protruding from the substantially central portion of the lens barrel 40.
Also, lens barrel holding bed 42 is formed with a through hole 45 into which lens barrel 40 is loosely inserted.
As shown in FIG. 5B, flange portion 44 of lens barrel 40 is placed on the supporting portion 42a of the lens barrel holding bed 42, and the flange portion 44 and the lens barrel holding bed 42 are fastened by means of bolts 43, whereby the lens barrel 40 is mounted on the lens barrel holding bed 42.
In the prior art described above, the lens barrel usually is mounted on the lens barrel holding bed while the environmental temperature is controlled to be a predetermined temperature.
However, the lens barrel and the lens barrel holding bed are formed of different materials and therefore, when for example, the environmental temperature changes due to the transportation or the like of the exposing apparatus, the lens barrel and the lens barrel holding bed make expansion and contraction differing by the difference between the coefficients of linear expansion of the respective materials.
Accordingly, when as in the prior art, the lens barrel is directly fastened and mounted on the lens barrel holding bed by means of bolts, if the environmental temperature changes, internal stress will be created in the lens barrel by the fastening of the bolts. The use of bolts has led to a problem that the optical performance of the projection optical system provided in the lens barrel is aggravated by such interval stress. However, the aggravation of the optical performance herein referred to so is that the imaging accuracy of the projection optical system is aggravated by the deviation of the optical axes of the optical elements in the lens barrel or variations in the refractive indices of the optical elements.